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The long-promised national bowel screening programme will fail to detect many cases of cancer because the threshold of the test will be set higher than in the pilot scheme, a screening expert says.

The screening age has already been restricted compared with both the Waitemata District Health Board pilot and Australia's national programme which is building up to all ages between 50 and 74 by 2020.

Now Herald inquiries have found the screening test will be set at a higher threshold than in Waitemata and Australia.

A leading cancer epidemiologist, Associate Professor Brian Cox of Otago University, said that as well as detecting a smaller percentage of cancers, the higher threshold will mean the cases found will probably be "more severe, more advanced and have a poorer survival". Fewer lives will be saved.

"It's almost a bowel screening programme that you try to have when you're not having one, or not having an effective one.

"You meet a political demand to have a screening programme and it's been whittled down because colonoscopy services can't meet the demand because of the particular screening test that they have decided to pursue despite it now being some 10 years out of date."

Internationally recognised experts on bowel screening were involved in the decisions on the choice of test as well as parameters for the New Zealand national bowel screening programme

New Zealand has one of the highest bowel cancer rates in the developed world. Each year around 3000 people are newly diagnosed with the disease and more than 1200 die from it. Patients tend to be diagnosed later in New Zealand than in Australia - and the UK, which also has a screening programme.

In the May Budget, the Government committed $39.3 million to expand the Waitemata scheme into a national screening programme, in three "tranches" of DHBs. Wairarapa and Hutt Valley are planned to be first, in the middle of next year, followed in 2018 by Auckland, Canterbury, Capital and Coast, Hawke's Bay, Southern, Taranaki, Waikato, West Coast and Whanganui; then, in 2019, Bay of Plenty, Counties Manukau, Lakes, MidCentral, Nelson Marlborough, Northland, South Canterbury and Tairawhiti.

Eligible people are sent a screening invitation and test kit every two years. They take tiny poo samples at home and post them in a special container for lab testing for invisible blood components that could indicate abnormalities. Positive results will generally lead to referral for colonoscopy - a diagnostic check of the large intestine with a flexible viewing tube.

The pilot's threshold is 15 micrograms of blood haemoglobin per gram of dried faeces. In Australia it is 20. In New Zealand's national programme it will be 40, the Health Ministry said.

"Outcomes in the national programme will be monitored very closely and there is a possibility that the threshold for positivity of the FIT [faecal immunochemical test] will be raised or lowered as required in the future," said Dr Susan Parry, the ministry's clinical director for bowel cancer.

"Each country has different requirements and some have thresholds that are higher than [ours]; some have lower. Our approach is consistent with Ireland and the Netherlands."

The key problem since a national programme was mooted more than a decade ago is that, despite expansion, New Zealand has too little colonoscopy capacity to consistently cope with patients with bowel cancer symptoms and post-treatment surveillance, let alone the wave of asymptomatic patients with a positive screening result.

The need to shrink that wave led to the planned age restriction - 60-74 in the national programme, compared with 50-74 in the pilot - and now the higher test threshold.

Waitemata data presented to an American conference by Parry and a colleague showed that increasing the test threshold from 15 to 40 micrograms/gram would have reduced the number of cancers detected by 17 per cent - and the number of colonoscopies by 43 per cent.

However, the ministry expects the national programme will detect more cancers for every thousand people screened than in Waitemata, partly because an older group has proportionally more cancers. Cox added that Waitemata's incidence rates of the disease had been lower than in most of the country before the pilot.

The ministry expects the programme will reduce bowel cancer deaths by 16-22 per cent in the screened age group within 10 years.

Acting Health Minister Peseta Sam Lotu-Iiga defended the screening plans, saying: "Internationally recognised experts on bowel screening were involved in the decisions on the choice of test as well as parameters for the New Zealand national bowel screening programme."

Cox wants New Zealand to offer a one off mini-colonoscopy test called flexible sigmoidoscopy, which he said would avoid the lack-of-capacity problems. The ministry disputes this.

Patient support and advocacy group Bowel Cancer NZ, which welcomes the commitment to a national programme, is disappointed by the test threshold increase.

It was hoping for a programme in which "as many people as possible are detected when the cancer is at an earlier stage and able to be treated with minimal intervention required and the best possible prognosis for patients", said spokeswoman Sarah Derrett.

"Inevitably some smaller proportion of people who actually have bowel cancer are going to be detected through the screening programme. We would hope that we would move into a position of at least parity with Australia as soon as possible."

Burden of bowel cancer• About 3000 people are newly diagnosed a year• More than 1200 die each year

The screening programme• Fully operational by 2019• 60-74-year-olds eligible• Home DIY screening test every two years• Tiny poo sample sent to lab in special container• A set level of blood triggers referral for diagnostic testing, usually colonoscopy

The issue• The age range has been shrunk• Now the blood threshold for colonoscopy has been raised